KVM: VMX: Correct asm constraint in vmcs_load()/vmcs_clear()

'error' is byte sized, so use a byte register constraint.

Acked-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: Avi Kivity <avi@redhat.com>
Signed-off-by: Marcelo Tosatti <mtosatti@redhat.com>